Electrochemical machining is a widely used technique for providing holes in difficult-to-machine metal parts. Generally, these processes involve using electrochemical force (as opposed to mechanical force) to disengage or deplate material from a workpiece.
A highly specialized adaptation of electrochemical machining, known as shaped-tube electrolytic machining, is used for drilling small, deep holes in electrically conductive materials. Shaped-tube electrolytic machining is a noncontact electrochemical-drilling process that distinguishes itself from all other drilling processes by its ability to produce holes with aspect ratios of up to 300:1. Being an electrochemical process, shaped-tube electrolytic machining is unaffected by either material hardness or toughness. It uses an acid-based electrolyte instead of the salt electrolytes normally incorporated in electrochemical machining. The use of acid electrolytes ensures that the metal sludge by-products from electrolytic deplating are dissolved and carried away as metal ions. This eliminates clogging of the electrolyte flow path around the electrode, an important feature when drilling deep holes. Shaped-tube electrolytic machining processes are discussed in more detail in Machining Data Book, vol. 2, pp. 11-71 to 11-75 (3rd ed. 1980); E. J. Weller, Nontraditional Machining Processes, pp. 109-13 (2nd ed. 1984); and G. F. Benedict, Nontraditional Manufacturing Processes, pp. 181-87 (1987).
Advances in jet engine technology have resulted in the need to machine super alloys and metals. The characteristics of these metals and the complex designs associated with jet engine hardware have posed machining problems which are beyond the capability of conventional machining processes. As a result, shaped-tube electrolytic machine processes have found particular applicability in the manufacture of aircraft engines. These processes are especially useful in drilling holes through turbine blades, buckets, vanes, and struts so that cooling liquid can be circulated through these components during turbine operation. Since such cooling is enhanced by turbulence within the cooling passages, there is little reason for finely machining these passages. Examples of the use of shaped-tube electrolytic machining processes in conjunction with aircraft engine manufacture are disclosed in U.S. Pat. Nos. 3,352,770 to Crawford et al., 3,352,958 to Andrews, 3,793,169 to Joslin, 3,805,015 to Andrews, and 4,088,557 to Andrews.
In recent years, shaped-tube electrolytic machining processes have also found application in the manufacture of precision extrusion dies for producing ceramic honeycomb structures. Such structures are particularly useful for automobile catalytic converters.
The manufacture of extrusion dies from these ultra-hard materials is an extremely tedious process. The extrusion dies are formed with multiple apertures through which the extrudate is forced under high pressure. In one method of forming the extrusion die, mechanical drills are used to provide the extrusion apertures. If the extrusion dies are formed of ultra-hard materials such as, for example, 17-4PH stainless steel or Inconel.RTM. 718 (a registered trademark of International Nickel Co., Inc.), the drilling rate used for aperture formation is very slow and a great deal of time and effort is expended in extrusion die formation. If softer die materials are used, the drilling rate is increased, but the life span of the resulting extrusion die is correspondingly shorter.
Because of these difficulties, apertures are now formed in extrusion dies by electrochemical machining techniques rather than mechanical drilling. With an electrochemical machining process, the workpiece from which the die is to be formed is situated in a fixed position relative to a movable manifold. The manifold supports a plurality of drilling tubes, each of which are utilized to form an aperture in the workpiece. The drilling tubes operate as cathodes in the electrochemical machining process, while the workpiece comprises the anode in that process. As the workpiece is flooded with an acid electrolyte from the manifold, material is selectively deplated from the workpiece in the vicinity of the drilling tubes to form the requisite aperture pattern. U.S. Pat. No. 4,687,563 to Hayes and European Patent Application Publication No. 0245 545 to Peters disclose such processes. Although this production technique has found significant usefulness in the art, the resulting extrusion dies can suffer from a problem of surface roughness in or near the holes.
In the use of shaped-tube electrolytic machining to manufacture extrusion dies, the presence of roughness in the passages of the dies is undesirable. Such roughness imparts friction against extrusion of material. These frictional forces are often so great that the ram for the extruder is unable to push material through the die. The present invention is directed to overcoming this problem.